126 research outputs found
Heavy Quark Production Asymmetries
In the hadroproduction of charm (or heavy flavours in general) in the context
of string fragmentation, the pull of a beam remnant at the other end of a
string may give a charm hadron more energy than the perturbatively produced
one. The collapse of a low-mass string to a single hadron is the extreme case
in this direction, and gives rise to asymmetries between leading and
non-leading charm hadrons. We study these phenomena within the Lund string
fragmentation model and improve the modelling in part by a consideration of
hadroproduction data. Applications include heavy quark production in any
collision between hadron-like particles such as \gamma p at HERA and pp at
HERA-B or the LHC.Comment: 6 pages, 6 figures, to appear in the Proceedings of the International
Europhysics Conference on High Energy Physics, Tampere, Finland, 15-21 July
1999, edited by K. Huitu, H. Kurki-Suonio and J. Maalamp
Production and Hadronization of Heavy Quarks
Heavy long-lived quarks, i.e. charm and bottom, are frequently studied both
as tests of QCD and as probes for other physics aspects within and beyond the
standard model. The long life-time implies that charm and bottom hadrons are
formed and observed. This hadronization process cannot be studied in isolation,
but depends on the production environment. Within the framework of the string
model, a major effect is the drag from the other end of the string that the c/b
quark belongs to. In extreme cases, a small-mass string can collapse to a
single hadron, thereby giving a non-universal flavour composition to the
produced hadrons. We here develop and present a detailed model for the
charm/bottom hadronization process, involving the various aspects of string
fragmentation and collapse, and put it in the context of several heavy-flavour
production sources. Applications are presented from fixed-target to LHC
energies.Comment: 40 pages, 25 figure
Drag Effects in Charm Photoproduction
We have refined a model for charm fragmentation at hadron colliders. This
model can also be applied to the photoproduction of charm. We investigate the
effect of fragmentation on the distribution of produced charm quarks. The drag
effect is seen to produce charm hadrons that are shifted in rapidity in the
direction of the beam remnant. We also study the importance of different
production mechanisms such as charm in the photon and from parton showers.Comment: 6 pages, 5 figures, Proc. of DESY Workshop "Monte Carlo Generators
for HERA Physics
The Sources of b-Quarks at the Tevatron and their Correlations
The leading-log order QCD hard scattering Monte-Carlo models of HERWIG,
ISAJET, and PYTHIA are used to study the sources of b-quarks at the Tevatron.
The reactions responsible for producing b and bbar quarks are separated into
three categories; flavor creation, flavor excitation, and
parton-shower/fragmentation. Flavor creation corresponds to the production of a
b-bbar pair by gluon fusion or by annihilation of light quarks, while flavor
excitation corresponds to a b or bbar quark being knocked out of the
initial-state by a gluon or a light quark or antiquark. The third source occurs
when a b-bbar pair is produced within a parton shower or during the
fragmentation process of a gluon or a light quark or antiquark (includes gluon
splitting). The QCD Monte-Carlo models indicate that all three sources of
b-quarks are important at the Tevatron and when combined they qualitatively
describe the inclusive cross-section data. Correlations between the b and bbar
quark are very different for the three sources and can be used to isolate the
individual contributions.Comment: RevTex4, 14 pages, 20 figures, submitted to Phys. Rev.
Transverse Momentum as a Measure of Colour Topologies
Several distinct colour flow topologies are possible in multiparton
configurations. A method is proposed to find the correct topology, based on a
minimization of the total transverse momentum of produced particles. This
method is studied for three-jet and four-jet events. It is shown how the basic
picture is smeared, especially by parton-shower activity. The method therefore
may not be sufficient on its own, but could still be a useful complement to
others, and e.g. help provide some handle on colour rearrangement effects.Comment: 1+8 pages, LaTeX2e, 6 eps figures included in file using filecontents
environment
QCD Radiation off Heavy Particles
We study QCD radiation in decay processes involving heavy particles. As
input, the first-order gluon emission rate is calculated in a number of
reactions, and comparisons of the energy flow patterns show a non-negligible
process dependence. To proceed further, the QCD parton shower language offers a
convenient approach to include multi-gluon emission effects, and to describe
exclusive event properties. An existing shower algorithm is extended to take
into account the process-dependent mass, spin and parity effects, as given by
the matrix element calculations. This allows an improved description of
multiple gluon emission effects off b and t quarks, and also off nonstandard
particles like squarks and gluinos. Phenomenological applications are presented
for bottom production at LEP, Higgs particle decay to heavy flavours, top
production and decay at linear colliders, and some simple supersymmetric
processes.Comment: 44 pages, 15 pages, 4 table
Production mechanisms of charm hadrons in the string model
In the hadroproduction of charm in the context of string fragmentation, the
pull of a beam remnant at the other end of a string may give a charm hadron
more energy than the perturbatively produced charm quark. The collapse of a
low-mass string to a single hadron is the extreme case in this direction, and
gives rise to asymmetries between charm and anticharm hadron spectra. We study
these phenomena, and develop models that describe the characteristics not only
of the charm hadrons but also of the associated event.Comment: 13 pages, 7 figures, submitted to Phys. Lett.
B Production Asymmetries in Perturbative QCD
This paper explores a new mechanism for B production in which a b quark
combines with a light parton from the hard-scattering process before
hadronizing into the B hadron. This recombination mechanism can be calculated
within perturbative QCD up to a few nonperturbative constants. Though
suppressed at large transverse momentum by a factor Lambda_QCD m_b/p_t^2
relative to b quark fragmentation production, it can be important at large
rapidities. A signature for this heavy-quark recombination mechanism in
proton-antiproton colliders is the presence of rapidity asymmetries in B cross
sections. Given reasonable assumptions about the size of nonperturbative
parameters entering the calculation, we find that the asymmetries are only
significant for rapidities larger than those currently probed by collider
experiments.Comment: 17 pages, LaTeX, 4 ps figures, tightenlines, sections added, final
version accepted for publication in Phys. Rev.
The Leading Particle Effect from Heavy-Quark Recombination
The leading particle effect in charm hadroproduction is an enhancement of the
cross section for a charmed hadron D in the forward direction of the beam when
the beam hadron has a valence parton in common with the D. The large D+/D-
asymmetry observed by the E791 experiment is an example of this phenomenon. We
show that the heavy-quark recombination mechanism provides an economical
explanation for this effect. In particular, the D+/D- asymmetry can be fit
reasonably well using a single parameter whose value is consistent with a
recent determination from charm photoproduction.Comment: Revtex file, 4 pages, 3 figure
- …